This paper presents the results of the interdisciplinary research project "Tolerance-free series production of high-performance concrete modules through transient-interactive coupling of design and production" within the priority program "SPP 2187", funded by the German Research Foundation (DFG). The project aims to enable tolerance-compensating, resource-efficient serial production of modular precast structures by integrating material behavior, structural design, manufacturing, and assembly. Methods for coupling design and production address final and intermediate construction states, assemblability, and sustainability. Placement strategies and specification heuristics link structural performance and assembly requirements with adaptive heat-treatment control. The production system is modeled using discrete-event simulation. Uncertainties are modeled probabilistically and the decision problem is formalized as a Markov Decision Process. Single- and multi-agent deep reinforcement learning systems select module-specific heat-treatment durations to reduce dimensional variability. A virtual demonstrator, complemented by experimental and pilot-scale structures, demonstrates the benefits of the integrated approach for precise, sustainable, industrialized precast concrete construction. Toleranzfreie Serienfertigung von Hochleistungsbetonbauteilen durch transient-interaktive Kopplung von Entwurf und ProduktionDieser Beitrag pr & auml;sentiert die Ergebnisse des interdisziplin & auml;ren Forschungsprojekts "Toleranzfreie Serienfertigung von Hochleistungsbetonbauteilen durch transient-interaktive Kopplung von Entwurf und Produktion" im Rahmen des von der Deutschen Forschungsgemeinschaft (DFG) gef & ouml;rderten Schwerpunktprogramms "SPP 2187". Das Projekt zielt darauf ab, eine toleranzausgleichende, ressourceneffiziente Serienfertigung modularer Fertigteilkonstruktionen zu erm & ouml;glichen, indem Materialverhalten, Tragwerksplanung, Fertigung und Montage integriert werden. Methoden zur Kopplung von Entwurf und Fertigung ber & uuml;cksichtigen End- und Zwischenzust & auml;nde der Konstruktion, Montagef & auml;higkeit und Nachhaltigkeit. Platzierungs- und Vorgabeheuristiken verkn & uuml;pfen die Tragwerksanforderungen mit einer adaptiven Steuerung der W & auml;rmebehandlung. Das Produktionssystem wird mittels diskreter Ereignissimulation abgebildet, und Unsicherheiten werden probabilistisch modelliert. Darauf aufbauend wird das Entscheidungsproblem als Markov-Entscheidungsprozess formalisiert ein Einzel- und Multi-Agenten Deep-Reinforcement-Learning-Systeme w & auml;hlen modul-spezifische W & auml;rmebehandlungsdauern aus, um Ma ss abweichungen zu reduzieren. Ein virtueller Demonstrator, erg & auml;nzt durch experimentelle Strukturen und Strukturen im Pilotma ss stab, veranschaulicht die Vorteile des integrierten Ansatzes f & uuml;r pr & auml;zises, nachhaltiges und industrialisiertes Betonfertigteilbauwesen.

The priority program (PP) 2187 "Adaptive Modular Construction made in a Flux" brings together the disciplines of civil engineering, mechanical engineering, architecture, and mathematics. The goal is replacing the craft-based manufacturing of concrete components with automated, serial production methods. Since 2020, 14 individual sub-projects - in addition to the coordination project - researched across two funding periods of 3 years each towards this paradigm shift in concrete construction. Thereby, 9 universities, 22 institutes, 25 principal investigators (PI) and around 60 early-career researches participated from all over Germany. The PP was initiated and coordinated by the Institute of Concrete Structures at the Ruhr University Bochum. The structure and organization of the PP was carried out through thematic working groups, benchmark- and demonstrator projects. In addition to annual status meetings, the research findings were published in national and international journals as well as at conferences presenting project-related and collaborative results. Furthermore, workshops were organized and conducted to provide discipline-specific and interdisciplinary education for participants. The present contribution outlines the key developments and measures undertaken.

The reuse of load-bearing reinforced concrete elements represents a promising approach to reducing CO2 emissions and resource consumption in the construction industry. This paper presents the holistic process of modular reuse, ranging from the identification of suitable donor structures through element extraction and characterization to the assembly of new load-bearing structures. Attention is paid to the development of systematic methods for element identification using Scan-to-BIM, as well as to the evaluation of different separation methods in terms of their impact on element quality. Experimental investigations combine non-destructive and semi-destructive testing methods to determine material properties and structural integrity. The results show that, although selective demolition methods require greater effort, they contribute significantly to the preservation of element quality. In addition, a digital element catalogue with a semantic data structure is presented, which enables consistent documentation and further processing of element information. The approaches developed are validated in full-scale demonstration projects and contribute to the establishment of circular construction methods. Modulare Wiederverwendung bestehender Tragstrukturen: Vom Konzept bis zur Demonstration im Originalma ss stabDie Wiederverwendung von tragenden Stahlbetonbauteilen stellt einen vielversprechenden Ansatz zur Reduzierung von CO2-Emissionen und des Ressourcenverbrauchs in der Bauindustrie dar. Dieser Beitrag stellt den ganzheitlichen Prozess der modularen Wiederverwendung vor, der von der Identifizierung geeigneter Spendergeb & auml;ude & uuml;ber die Entnahme und Charakterisierung der Bauteile bis hin zum Aufbau neuer Tragkonstruktionen reicht. Ein Schwerpunkt liegt auf der Entwicklung systematischer Methoden zur Elementidentifizierung mittels Scan-to-BIM sowie auf der Bewertung verschiedener Trennverfahren hinsichtlich ihrer Auswirkungen auf die Elementqualit & auml;t. Experimentelle Untersuchungen kombinieren zerst & ouml;rungsfreie und halbzerst & ouml;rende Pr & uuml;fverfahren zur Bestimmung der Materialeigenschaften und der strukturellen Integrit & auml;t. Die Ergebnisse zeigen, dass selektive Abbruchverfahren zwar einen h & ouml;heren Aufwand erfordern, aber wesentlich zur Erhaltung der Elementqualit & auml;t beitragen. Dar & uuml;ber hinaus wird ein digitaler Elementkatalog mit einer semantischen Datenstruktur vorgestellt, der eine konsistente Dokumentation und Weiterverarbeitung von Elementinformationen erm & ouml;glicht. Die entwickelten Ans & auml;tze werden in Demonstrationsprojekten im Realma ss stab validiert und tragen zur Etablierung zirkul & auml;rer Bauweisen bei.

During their life cycle, prestressed concrete bridges are prone to wire breaks of their tendons due to fatigue or stress corrosion cracking. However, not all structures show visible cracking on the surface, which would provide timely warning of otherwise invisible damage. Brittle failure may occur. Continuous monitoring with suitable techniques offers a remedy. This paper proposes embedded ultrasonic sensors and coda wave interferometry, which detect strain changes in the concrete caused by losses of prestress via acoustoelastic changes in the ultrasonic wave velocity. Preliminary tests on beams have demonstrated the general suitability of the method for localization. Building on this, the contribution now takes decisive steps further: in full-scale tests on a post-tensioned T-beam, six prestressing strands were successively cut through-initially without, and then with accompanying crack formation. A spatial sensor network yields 2D localization maps that correlate with strain fields from distributed fiber optic sensors (DFOS) and crack patterns from digital image correlation (DIC). Prestressing wire breaks in non-cracked concrete are reliably localized across the height and length, and consistently quantified. In cracked members, however, decorrelation of the signals due to cracking dominates the strain changes. Then detection becomes essentially limited to the cracked region. Finally, the impact of the sensor distance on localization accuracy is discussed.

Rising global construction demand calls for greater economic efficiency and ecological sustainability. Precast concrete construction (PCC) can reduce on-site errors and waste by using controlled production environments. However, PCC remains limited by traditional, labour-intensive methods ill-suited to automation and individualised elements. Advances in digital fabrication enable customised concrete modules at scale, but realising this potential requires consistent digital representations that integrate design and production. This paper presents an integrated framework leveraging Industry 4.0 concepts to overcome these limitations, specifically employing the Asset Administration Shell (AAS) to implement modular Digital Twins (DTs). Drawing on perspectives from multiple disciplines, this research outlines design and optimisation methods that demonstrate the potential for highly differentiated, precise concrete modules from various digital production processes. Based on a conceptual multi-storey building as a case study, this work explores advances in the design and production of precast concrete modules to highlight the diverse requirements and use cases for DTs in PCC. Three DT case studies are developed, which support design, production, and quality control. These include the simulation-based geometric modularisation of building elements to support early design phases, the monitoring and structuring of production data for analytical insights, and the management of geometric deviations of individual building modules, assessed in relation to their cumulative effect on the overall structural assembly. The results demonstrate the feasibility and effectiveness of integrating the DT concept via the AAS to manage complexity across design and production phases of individualised precast structures, paving the way for more sustainable and efficient construction practices in concrete.

Modular construction with precast concrete elements offers significant potential for rapid and resource-efficient building. This paper presents findings from the scientific coordination project of Priority Program (PP) 2187, addressing key challenges in the design and production chain. Generalized structural models for 1D and 2D modular structures are derived, and load-bearing capacities of slender reinforced concrete modules are described using M/N interaction charts including geometric scatter. Production-induced tolerances are systematically quantified and propagated to the structural level via Gaussian error propagation. For multi-dimensional structures, a sensitivity-based placement strategy using Elementary Effects is proposed, reducing global deformations by up to 43%. Construction sequences are optimized using Simulated Annealing, ensuring intermediate construction states do not govern final dimensioning. The convertibility of modular structures is assessed through sensitivity analysis of load redistribution. All boundary conditions are integrated into a digital twin based on the Asset Administration Shell. The findings reveal that adaptive modular construction represents a viable pathway toward industrialized, sustainable, and circular concrete construction. Adaptive Modulbauweisen mit Flie ss fertigungsmethodenModulares Bauen mit Betonfertigteilen bietet erhebliches f & uuml;r eine schnelle und ressourceneffiziente Bauweise. Dieser Beitrag pr & auml;sentiert Ergebnisse des wissenschaftlichen Koordinationsprojekts des Schwerpunktprogramms (SPP) 2187 und behandelt wesentliche Herausforderungen aus dem Entwurf und der Produktionskette. Es werden verallgemeinerte Tragwerksmodelle f & uuml;r modulare 1D- und 2D-Strukturen hergeleitet. Die Tragf & auml;higkeiten schlanker Stahlbetonmodule werden mithilfe von M/N-Interaktionsdiagrammen unter Ber & uuml;cksichtigung geometrischer Streuungen beschrieben. Produktionsbedingte Toleranzen werden systematisch erfasst und mittels Gau ss scher Fehlerfortpflanzung auf die Strukturebene & uuml;bertragen. F & uuml;r mehrdimensionale Tragwerke wird eine sensitivit & auml;tsbasierte Platzierstrategie mithilfe Elementarer Effekte vorgeschlagen, die globale Verformungen um bis zu 43% reduziert. Baureihenfolgen werden mithilfe von Simulated Annealing optimiert, sodass Bauzust & auml;nde nicht f & uuml;r die Bemessung ma ss gebend werden. Dar & uuml;ber hinaus wird die Wandelbarkeit modularer Strukturen durch Sensitivit & auml;tsanalysen der Lastumverteilung bewertet. Alle Randbedingungen flie ss en in einen digitalen Zwilling auf Basis der Verwaltungsschale ein. Die Ergebnisse zeigen, dass adaptives, modulares Bauen eine vielversprechende Methode f & uuml;r eine industrialisierte, nachhaltige und kreislauff & auml;hige Betonbauweise darstellt.

The construction sector faces major challenges in terms of resource efficiency and environmental impact. To address the lack of sustainability in infrastructural development, this paper proposes an innovative concept for the reuse of aged reinforced concrete elements. The aim is to enable the selective reconditioning, segmentation and reintegration of concrete components into new structures through an automated production system. The system combines advanced automation technologies with suitable connection designs and comprehensive quality assurance methods to ensure structural integrity and long-term performance. By applying a cradle to cradle philosophy and establishing a closed-loop supply chain for concrete elements, the approach significantly reduces waste, conserves natural resources, and lowers CO2 emissions. Building on this foundation, the paper reviews the state of the art in fine processing of aged concrete elements, with a focus on connection designs, automated processing, and quality assurance as key enablers of sustainable manufacturing and resilient infrastructure.

Long-term behavior of concrete structures depends on various factors, including loading history. Well- structured long-term experiments are essential to study the effects of loading history. Over the past two decades, research at FCE Skopje has focused on understanding the influence of realistic loading histories on the long-term behavior of full-scale elements made of normal-strength, high-strength, prestressed, and steel fiber-reinforced concrete. This paper presents selected results from four experimental programs on RC elements measuring 15/28/300 cm, tested under four-point bending. Using a specialised test setup, the elements were subjected to permanent and variable repeated loads applied in alternating cycles lasting several hours. Results collected over a year show that variable loads can significantly influence the long-term behavior regardless of the concrete type. Additional creep deformations depend more on load intensity than the duration of the applied load, while load pattern has minimal effect provided the maximum variable load intensity is included in the loading history.

The construction sector significantly contributes to global greenhouse gas emissions, primarily from cement production. To mitigate this, a strategy focusing on reusing structural components from existing reinforced concrete structures is being explored. This study highlights challenges and presents initial results in designing new structures from reused elements. The objective is to develop methods for designing load-bearing structures using available elements from demolished buildings, categorized in a construction kit. The challenge is to find a structure that meets load-bearing capacity and architectural demands under the constraints of available elements. The feasibility of integrating existing foundations into the design process is investigated. Non-destructive measurements and simulations characterize the foundation and soil properties, while methods for strengthening or adjusting the foundation are developed. The design process considers the constraints of the foundation and construction kit, with the coordinated arrangement of reused elements and connection types controlling stress distribution. The structural reliability of the proposed structure is assessed, quantifying the effect of uncertainties related to individual elements. Modulare Strukturen aus wiederverwendeten Bauteilen: Herausforderungen bei der Nutzung von Bestandsgr & uuml;ndungenDie Bauwirtschaft tr & auml;gt durch die Zementproduktion erheblich zu den globalen Treibhausgasemissionen bei. Zur Reduktion soll eine Strategie zur Wiederverwendung von Stahlbetonbauteilen vorgestellt werden. In diesem Beitrag wird die Entwicklung von Methoden f & uuml;r den Tragwerksentwurf unter Verwendung verf & uuml;gbarer Bauteile, die aus abzurei ss enden Geb & auml;uden entnommen wurden und in einem Baukasten-System kategorisiert sind, pr & auml;sentiert. Die zentrale Herausforderung besteht darin, eine Struktur zu finden, die eine ausreichende Tragf & auml;higkeit aufweist und den architektonischen Anforderungen gen & uuml;gt. Hierbei sollen bestehende Fundamente in den Entwurfsprozess integriert werden. Dazu werden zerst & ouml;rungsfreie Messungen mit Simulationen kombiniert, um die Eigenschaften von Fundament und Boden zu charakterisieren. Weiterhin wird die M & ouml;glichkeit einer Verst & auml;rkung durch Biozementierung untersucht. Im optimierungsgesteuerten Entwurfsprozess werden die Fundamente und verf & uuml;gbaren Bauteile als Randbedingungen ber & uuml;cksichtigt, wobei der Kraftfluss durch die gezielte Anordnung der Bauteile und entsprechender Verbindungstypen gesteuert werden kann. Die Zuverl & auml;ssigkeit der abgeleiteten Tragstrukturen wird durch nichtlineare Simulationen bewertet und Unsch & auml;rfen werden bez & uuml;glich des Bauteilzustands quantifiziert.

This study develops a Bayesian, uncertainty-aware framework for tendon breakage localization in pre-stressed concrete members using high-resolution data from distributed fiber-optic sensors (DFOS). DFOS enable full-field monitoring of strain changes on the surface of pre-stressed concrete members due to such failure. A finite element model (FEM) of an experimental tendon-breakage test is constructed, and model parameters are calibrated probabilistically against DFOS measurements. To capture model-form uncertainty (MFU), stochastic perturbations are embedded directly into material parameters, enabling the joint inference of physical properties and MFU within a unified probabilistic framework. Gaussian Process surrogates are employed to efficiently emulate the nonlinear FEM response, supporting computationally tractable Bayesian inference. A ϕ-divergence-based influence analysis identifies the DFOS measurements that most strongly shape the posterior distributions, providing interpretable diagnostics of sensor informativeness and model adequacy. The calibrated parameters and embedded uncertainties are then transferred to a FEM of a full-scale structural configuration, enabling prediction of tendon breakage localization under realistic conditions. A separability analysis of the predictive strain distributions quantifies the identifiability of tendon breakage at varying depths, assessing the confidence with which different damage scenarios can be distinguished given the propagated uncertainties. Results demonstrate that the framework achieves robust parameter calibration, interpretable diagnostics, and uncertainty-informed damage detection, integrating experimental data, embedded MFU, and probabilistic modeling. By systematically propagating both experimental and model uncertainties, the approach supports reliable tendon breakage localization and optimal DFOS placement.

Effective bridge management depends on transforming heterogeneous infrastructure and monitoring data into actionable intelligence. A data-driven workflow is presented for the prestressed concrete bridge stock in North Macedonia. Three datasets are utilised: (i) a national inventory of 1,310 structures, from which 122 precast prestressed concrete T-girder bridges are classified into four cross-sectional families using agglomerative clustering; (ii) dynamic load testing of 27 bridges with 146 controlled passages at 10-80 km/h; and (iii) a 30-day Bridge Weigh-in-Motion (B-WIM) campaign recording 22,006 heavy-vehicle events, segmented into eight vehicle typologies. B-WIM analysis identifies an inverse relationship between vehicle weight and Dynamic Amplification Factor (DAF), with 4-axle trucks showing the strongest association with upper-tail dynamic amplification in the monitored dataset. The bridge inventory analysis indicates that four clusters consolidate into two broader groups. DAF is influenced by multiple interacting parameters, and sensitivity analysis shows that the choice of signal-processing cut-off alone accounts for variations of up to 18.5 % at highway speeds. The envelope-based lower cut-off exhibits the strongest agreement with reference measurements, establishing a reliable extraction method for DAF. This workflow offers an empirical basis for defining bridge and vehicle typologies applicable to similar inventories, subject to local validation.

Ensuring geometric accuracy in precast concrete production is becoming increasingly critical, especially for modular, tolerance-sensitive designs. This is particularly evident in segmental structures with dry joints, where even minor deviations can significantly affect assembly and structural performance. Within this context, a demonstrator for Digital Twin (DT)-based quality monitoring is developed using a modular precast segmental pedestrian bridge as a test case. High-resolution geometric data for individual segments are acquired via structured-light scanning and linked to digital representations of the segments. These are subsequently integrated into a DT of the overall structure, enabling the combined consideration of design and as-built information. The demonstrator illustrates how scan-based data can support the identification of geometric deviations and contribute to quality-related decision-making during production. In addition, the integration of measurement data into a DT environment is outlined, highlighting opportunities for improved traceability and consistency across production stages. Qualit & auml;tskontrolle auf Basis Digitaler Zwillinge: Sicherung der Pr & auml;zision bei BetonfertigteilenDie Gew & auml;hrleistung der geometrischen Genauigkeit bei der Herstellung von Betonfertigteilen gewinnt zunehmend an Bedeutung, insbesondere bei modularen, toleranzempfindlichen Konstruktionen. Dies zeigt sich besonders deutlich bei Segmentkonstruktionen mit Trockenfugen, bei denen bereits geringf & uuml;gige Abweichungen die Montage und die statische Leistungsf & auml;higkeit erheblich beeintr & auml;chtigen k & ouml;nnen. Vor diesem Hintergrund wird ein Demonstrator f & uuml;r die Digitale-Zwillings (DT)-basierte Qualit & auml;ts & uuml;berwachung entwickelt, wobei eine modulare Fu ss g & auml;ngerbr & uuml;cke aus Betonfertigteilen als Testfall dient. Hochaufl & ouml;sende geometrische Daten f & uuml;r einzelne Segmente werden mittels Streifenlicht-Scanning erfasst und mit digitalen Darstellungen der Segmente verkn & uuml;pft. Diese werden anschlie ss end in einen DT der Gesamtkonstruktion integriert, wodurch die kombinierte Ber & uuml;cksichtigung von Konstruktions- und Bestandsdaten erm & ouml;glicht wird. Der Demonstrator veranschaulicht, wie scanbasierte Daten die Identifizierung geometrischer Abweichungen unterst & uuml;tzen und zur qualit & auml;tsbezogenen Entscheidungsfindung w & auml;hrend der Produktion beitragen k & ouml;nnen. Dar & uuml;ber hinaus wird die Integration von Messdaten in eine DT-Umgebung skizziert, wobei M & ouml;glichkeiten f & uuml;r eine verbesserte R & uuml;ckverfolgbarkeit und Konsistenz & uuml;ber alle Produktionsstufen hinweg aufgezeigt werden.

This study investigates the numerical modeling of internal tendon breaks in prestressed concrete structures and evaluates the key parameters affecting accuracy. A combined approach is followed, involving global sensitivity analysis via the Elementary Effects Method and subsequent model calibration using experimental data from distributed fiber-optic sensors. A finite element model is implemented in ABAQUS, using the non-linear Concrete Damage Plasticity constitutive law and accounting for geometrical non-linearity by explicitly modeling the bond between concrete and tendon. The sensitivity study identifies four critical parameters, namely Young's modulus of concrete, friction coefficient, initial contact pressure, and contact clearance, as most influential on the surface strain field. These are calibrated using strain measurements from tendon break experiments on beams with varying geometries employing different concretes and tendon types. Based on the calibration results the computational model is geometrically scaled to a full-size box girder. The results demonstrate how prestressing level, tendon depth, and bond characteristics influence the detectability of internal tendon breaks and provide valuable insights for optimizing numerical models used in structural health monitoring of existing prestressed concrete structures.

Adaptive modular construction methods enable resource-efficient, rapid construction. Their implementation requires precise, quality-assured prefabrication. However, available suitable prefabrication facilities are still rare in the construction industry. This contribution introduces a research facility for automated production of adaptive reinforced concrete modules. The facility is equipped with three industrial robots and covers the process chain of reinforcement, formwork, and rapid curing with integrated quality assurance controls. In a cooperative process involving two robots, reinforcement elements are welded automatically, enabling all standard reinforcement layouts. The developed formwork system allows for variable cross-sections and surfaces (adaptability) with dimensional deviations of less than +/- 2 mm. In-process geometric and mechanical inspections using a laser line sensor, stereo camera, and a rebound hammer serve for quality assurance and can be used to control subprocesses. A curing chamber with controllable temperature and humidity conditions accelerates the curing of the concrete to completion in as little as 1 hour. During and after construction of the physical facility, a digital facility model enables simulation and virtual commissioning. Robotergest & uuml;tzte Flie ss fertigung modularer Betonbauteile: Forschungsanlage und digitales ModellAdaptive Modulbauweisen erm & ouml;glichen ressourcenschonendes und schnelles Bauen. Die Umsetzung erfordert eine pr & auml;zise, qualit & auml;tsgesicherte Vorfertigung. Verf & uuml;gbare geeignete Vorfertigungen sind im Bauwesen jedoch noch selten. Im Beitrag wird eine Forschungsanlage vorgestellt, in der automatisierte Prozesse zur Fertigung adaptiver Stahlbetonmodule umgesetzt werden. Die Anlage ist mit drei Industrierobotern ausgestattet und bildet die Prozesskette aus Bewehren, Schalen und Schnellh & auml;rtung mit integrierten Kontrollen zur Qualit & auml;tssicherung ab. Im kooperativen Prozess zweier Roboter werden Bewehrungselemente automatisiert geschwei ss t, was alle & uuml;blichen Bewehrungsf & uuml;hrungen erm & ouml;glicht. Das entwickelte Schalungssystem erlaubt variable Querschnitte und Oberfl & auml;chen (Adaptivit & auml;t) mit Ma ss abweichungen unter +/- 2 mm. Prozessbegleitende geometrische und mechanische Kontrollen mittels Laserliniensensor, Stereokamera und R & uuml;ckprallhammer dienen der Qualit & auml;tssicherung und sind zur Regelung von Teilprozessen nutzbar. Eine H & auml;rtekammer mit regelbaren Temperatur- und Feuchtebedingungen beschleunigt die Erh & auml;rtung des Betons auf Dauern bis 1 h. Sp & auml;ter und schon w & auml;hrend der Erstellung der physischen Anlage dient ein digitales Anlagenmodell der Simulation und virtuellen Inbetriebnahme vorab.

Thermal prestressing of supplemental reinforcement – experimental implementation on unloaded beams Conventional strengthening measures for load-bearing structures are usually not effective for the self-weight. This paper presents a self-weight-effective strengthening method for reinforced concrete structures subjected to bending using additional thermally prestressed reinforcement. Reinforcing bars, slotted into the tensile zone and embedded into filler material, are tempered from the outside and thus thermally expanded. If the heat supply is stopped, the bond with the filler material, which is hardened in the meantime, prevents the bars from deforming when cooling. The hereby induced prestress counteracts the self-weight and relieves the structure. This makes such supplemental reinforcement effective for the self-weight. The paper shows an experimental implementation of this strengthening method on unloaded beams, measures and quantifies the prestress induced, and validates an analytic computational model derived for the strengthening method. Slot widths of 8 and 10 cm are examined competitively under otherwise identical conditions. Moreover, the most influential factors of effectiveness of thermal prestressing are discussed. The results prove the general feasibility of structural strengthening by thermal prestressing of supplemental reinforcement. By the method, the bars are pre-strained by approx. 0.5 ‰ which is even better than analytically predicted. This is attributed to an additional prestressing effect of the thermally expanded filler material. With the slot width the tempered area rises, but the effectiveness of strengthening is reduced by heating the entire cross-section. On top, creeping of the bond impairs the prestress, too, while shrinkage of the filler material increases it vice versa.

Fatigue strength verification using structurally specific S-N curves – A proposal for discussion Prestressed concrete bridges exposed to fatigue require special attention. Their safety must be assessed and, if necessary, continuously monitored. An indicator that always announces failure in good time appears suitable. The highest verification level acc. to DIN EN 1992-1-1 (EC2) provides for an explicit fatigue strength verification. However, the available S-N curves are not very suitable for this purpose. They apply integrally to all approved prestressing steels and systems and do not adequately cover specific applications. It is suggested that fatigue tests are carried out on prestressing steel in air that largely corresponds to the installed material. This leaves only the transfer to bonded tendons, for which a solution is proposed. Using the interactive approach, experimental cycles to failure are converted into a S-N curve with inflection point, then transformed into a bilinear curve affine to EC2 and compared with a general reference for prestressing steel in air. Its positional offset to the S-N curve of straight post-tensioned tendons is transferred to the specific curve to make it usable for the reference structure. It is far more favorable than the normative one and significantly extends the remaining service life. It would be worth considering making the results of fatigue tests in the course of approvals a binding part of the structural documentation in future.

Conventional strengthening measures for existing structures are usually not effective for the self-weight, which accounts for around 70% of the total load in reinforced concrete structures. Therefore, their effect on the overall load-bearing capacity is low. A self-weight-effective alternative for flexural strengthening is the thermal prestressing of additional reinforcement installed on the structure. In this method, reinforcing bars are slotted into the tensile zone, embedded in filler material, and tempered from the outside. They are thermally stretched, and once cooling starts, the bond with the hardened filler prevents re-deformation. The induced prestressing force counteracts dead loads and relieves the tensile zone, making the additional bars effective for the self-weight. In this paper, the effectiveness of the strengthening method is experimentally investigated in the serviceability and the ultimate limit states. Experiments involve strengthening a reinforced concrete beam under load by a thermally prestressed additional bar. Moreover, two reference tests are made to evaluate the method. An unstrengthened beam characterizes the lower capacity limit. Another beam with the same reinforcement amount as the strengthened one, but completely installed at casting, serves as the upper benchmark. All beams are loaded until bending failure. The strengthening method is assessed by means of the load-bearing behavior, deflection, crack development, and the strains in the initial as well as the added reinforcement. The results demonstrate the effectiveness of the strengthening method. The thermally prestressed bar achieves an effective pre-strain of approximately. 0.4‰ by heating at about 70 °C. The induced prestressing force and associated compression reduce tensile cracks by approx. 45% and increase stiffness. The strengthened beam reaches the maximum load of the upper benchmark, but with about 33% less deflection. The filler, which also expands thermally, generates an additional prestressing force that is effective up to about 20% of the load capacity. Beyond this, the filler begins to crack and its effect decreases, but the pre-strain in the reinforcing bar remains until maximum load.

Steel fiber reinforced concrete (SFRC) is widely used in structural applications due to its residual tensile strength, crack control and durability. Its performance can be optimized through two strategies: 1) positioning a higher-performance SFRC only in the zones of maximum stress and 2) orienting fibers to align with principal stress directions. This study introduces a novel optimization strategy to enhance SFRC structural elements: the above mentioned optimization is carried out through an innovative casting device and methodology that ensure directional control of fiber alignment, leveraging the material's anisotropic properties, and allow casting different SFRC materials in a monolithic structural element. The effectiveness of this approach is assessed through six full-scale plates subjected to concentrated loads, three with standard reinforcement solutions and three optimized SFRC specimens, including curved strips of high-performance SFRC with oriented fibers, following the tension isostatic lines. Results indicate that optimized SFRC specimens exhibit up to a threefold improvement in crack control with respect to traditional rebar reinforcement and a 40 % increase in bearing capacity as compared to traditional SFRC. Additionally, direct comparison of residual performance and fiber orientation - measured with an electromagnetic induction method - is carried out between standard beams (cast according to EN 14651) and beams cut from the tested plates, showing the effectiveness of the casting device in steering fiber orientation and suggesting a good correlation between performance and orientation.

With regard to climate change and the importance of utilization of solar energy, the development of a modular concrete heliostat is presented. The focus thereby lies on the design and the construction of the concentrator structure, demonstrating the technical proof of concept fora small-scale collector. The idea of using concrete as a structural material is its free shapeability, and its worldwide availability. With respect to accuracy demands, a high-performance concrete (HPC) is used that possesses a high compressive and also tensile strength. The collector is designed as a strut-like structure with main radial beams and a central mount to ensure high stiffness. A circular design minimizes shading effects in the solar field. By employing symmetry reduction methods, the concentrator is dissolved into equal segments making it a modular construction. To demonstrate the feasibility, a prototype with diameter 3.2 m and a weight of just about 340 kg consisting of four modules was developed. The production is achieved using a modular formwork made from polystyrene at the RPTU Kaiserslautern. The concrete heliostat is built-up and qualified at the solar tower J & uuml;lich (Germany) by means of photogrammetric measurement of the mirror surface. In addition, the concrete structure was measured in the lab of the Ruhr University Bochum. The deformations of the concrete structure vary mainly in the range of +/- 1 mm only indicating remarkable stiffness. In contrast, the mirror deformations reveal an optical efficiency of SDrms = 7.8 mrad. However, these deformations are primarily attributed to the simple mirroring concept by means of clamping, which was not the central subject of the development. However, deviations between varying collector positions are less than 2 mm and only occur in local areas of single mirror facets, which additionally underlines the stiffness of the concrete structure.